Electrophotography is a dry copy process whereby copies of documents are made by setting up an electrostatic image of an original document and transferring that image to copy paper. Also known as xerography, electrophotography has become a standard process for creating copies of documents in a host of environments including offices, educational institutions and the like. The fundamental principles of electrophotography are well known to those skilled in the art.
In more recent years, various systems for color electrophotography have been created. In many respects, the process of color electrophotography is analogous to standard three-color printing processes used in the more conventional printing arts. Conventional three-color printing component images, commonly referred to as color separations, are created by photographing the original through appropriate filters. Each of the separations is in turn made into a separate printing plate. During the printing process, each plate is inked with an appropriate color determined by the filter used in making the original separation. The printing press is adjusted for proper registration--alignment of the separate color component images with each other. Once the press is properly adjusted, multiple copies of the original color image may be faithfully reproduced.
As is known to those skilled in the art, in monochromatic electrophotography an optical image of an original to be copied is focused onto an electrostatic medium. An electrostatic image of the original is formed on the electrostatic medium, or photoreceptor. Toner materials are then brought into contact with the photoreceptor and held in place on the appropriate portions of the electrostatic image by electrostatic forces. The photoreceptor carrying the electrostatic image is brought into contact with an image receptor which, in the most common applications of electrophotography, is a sheet of paper. Electrostatic charging techniques are used to transfer the toner from the photoreceptor to the image receptor. The toner materials are normally plastics which melt at a predetermined temperature and have appropriate color characteristics once they are melted. The image receptor with these toner particles is passed through a fuser, which is a station in the path of the image receptor. The fuser heats the transferred toner and thereby fixes the image onto the image receptor.
In color electrophotography, three separator color filters are typically used to create three separate color component images in a manner analogous to the creation of color separation in color printing. Each image is developed with a toner having the appropriate color characteristics. Each developed color component image is in turn transferred to the image receptor and overlaid upon the previous image to provide a composition image. The image receptor, typically paper, carrying the composite image is then passed through a fuser in a conventional manner.
It is known in the art of color electrophography to include an intermediate transfer medium upon which each developed color component image is deposited, between the above-described photoreceptor and the ultimate image receptor or paper. In this way, a composite developed image is built up, one color component image at a time, until an overlaid composite color image, having portions of all three of the color component toners thereon, is created on the transfer medium. Once this is accomplished, the composite image on the transfer medium is transferred to the paper which then passes through the fuser in the normal fashion. Examples of such systems are shown in U.S. Pat. Nos. 4,652,115 and 4,705,385, both entitled "PRINT ENGINE FOR COLOR ELECTROPHOTOGRAPHY", both incorporated by reference.
It is known in the art to use endless photoreceptive belts to serve as the above-discussed photoreceptors. It is also known to use endless belts for the above-discussed image receptors. These belts are known as photoreceptor belts, and image receptor belts, respectively. The term "endless" is used to denote belts which have no free ends. Such endless belts may be in the form of "seamless" belts which have no identifiable seam, or "seamed" belts which are essentially a length of belt having its end fashioned together by means known in the art, such that the belt is endless, but also has an identifiable seam.
Although such endless belts have several advantages, disadvantages exist in that they may tend to stretch over a period of time. Therefore, a need exists to provide a type of belt tensioning device which will prevent slack from developing in a belt, which results in improper registration, and a risk of damage to the machine.
A need also exists in the art for a device which will allow such endless belts to be readily and withdrawn from the electrophotographic system. As the belts are maintained in tension during operation, it is therefore necessary to provide a device which can quickly release tension on the belt, allowing it to be drawn away from the rollers, and thereafter replaced with the same or a different belt.
From the foregoing, it will be appreciated that a need exists in the art for improved belt tensioning device which maintains desired tension on an endless belt, and also allows endless belts to be readily withdrawn or installed into the device.